Perfluoropolyether carboxylic acid fluoride F(CF2CF2CF2O)n-CF2CF2COF can be produced by ring-opening polymerization of 2,2,3,3-tetrafluorooxetane in the presence of an alkali metal fluoride to form F(CH2CF2CF2O)nCH2CF2COF, followed by fluorination with a fluorine gas.
Patent Literature 1: JP-B-63-32812
Patent Literature 2: JP-B-63-43419
Hydrogen fluoride HF is formed as a by-product by the reaction, and when the reaction temperature is elevated in the presence of such hydrogen fluoride, decomposition of etheral bond parts will take place to lower the degree of polymerization or the yield. To suppress such a decomposition it is necessary to lower the reaction temperature. The starting material F(CH2CF2CF2O)nCH2CF2COF is in a waxy or solid state at the ordinary temperature, depending on a degree of polymerization n, whose melting point is as high as about 80° C. To conduct the fluorination in a liquid state with a fluorine gas, the reaction temperature must be made higher.
This means that a much higher reaction temperature will be required with increasing degree of polymerization, and thus even if a starting material having a much higher degree of polymerization is used, the decomposition reaction is proceed in the end, so it is difficult to synthesize perfluoropolyether carboxylic acid fluoride having a desired higher degree of polymerization in good yield.
It is conceivable to use a solvent to conduct the reaction gently as a means of suppressing the decomposition reaction, which takes place during the fluorination reaction. So far available solvents for dissolving the starting material tetrafluorooxetane polymers are polar solvents, but the high polymers are hard to dissolve even in such solvents. At higher reaction temperatures, the solvents themselves can react with the fluorine gas and thus are hard to use in the fluorination reaction.
To trap hydrogen fluoride, which is caused the decomposition reaction, it is conceivable to conduct the fluorination in the presence of an alkali metal fluoride, but 2 molecules of hydrogen fluoride can be formed as a by-product per repeat unit —CH2CF2CF2—, and to comply trap the hydrogen fluoride a large amount of alkali metal fluoride must be used with respect to the starting material. Furthermore, even if an equivalent weight of an alkali metal halide to the hydrogen fluoride is used, stirring will be hard to conduct, so smooth reaction will be hard to attain, and the yield will be inevitably lowered due to removal of the alkali metal halide by filtration, etc.
According to the afore-mentioned Patent Literature 2, the desired product can be obtained by conducting ultraviolet irradiation during the reaction to elevate the reactivity of hydrogen fluoride gas, thereby lowering the reaction temperature to about 100° to about 120° C. However, no examples of synthesizing perfluoropolyether carboxylic acid fluoride having a high degree of polymerization (n=about 40) are disclosed therein and the disclosed procedure of ultraviolet irradiation will require lamps of larger size, when the reactor is to be scaled up, also with a higher running cost, and thus is not suitable for the industrial scale production.